Graphene-augmented composite materials

ABSTRACT

Composite materials having carbon reinforcement fibers impregnated with a matrix material are augmented with functionalized graphene nanoplatelets having amine groups formed on a surface of the graphene nanoplatelets and epoxide groups formed on at least one edge of the graphene nanoplatelets as a supplement to or a replacement for resin matrix material to increase strength of the composite materials. Related methods of increasing strength of composite materials include mixing the functionalized graphene nanoplatelets into the matrix material prior to impregnating the carbon reinforcement fibers, depositing the functionalized graphene nanoplatelets onto the matrix material to form an interlayer, and depositing the functionalized graphene nanoplatelets onto a bed of carbon reinforcement fibers with no resin matrix material. The composite materials and related methods for increasing strength of composite materials may include graphene nanoplatelets having holes formed through the graphene nanoplatelets.

RELATED PATENT APPLICATION

This application is a continuation of and claims priority from U.S.patent application Ser. No. 14/589,007 filed on Jan. 5, 2015 now U.S.Pat. No. 10,266,677.

TECHNICAL FIELD

This disclosure generally relates to composite materials augmented withgraphene, composite structures including component parts made fromgraphene-augmented composite materials, and methods for manufacturinggraphene-augmented composite materials and composite structures withcomponent parts made from graphene-augmented composite materials.

BACKGROUND

Composite materials are used in the manufacture of a wide variety ofstructures and component parts due to their high strength and rigidity,low weight, corrosion resistance and other favorable properties. Forexample, in the aerospace industry, composite materials are becomingwidely used to manufacture aerospace structures and component parts foraerospace structures such as aircraft ribs, spars, panels, fuselages,wings, wing boxes, fuel tanks, tail assemblies and other component partsof an aircraft because they are lightweight and strong, and thereforeprovide fuel economy and other benefits.

One type of composite material commonly used in the aerospace industryis carbon fiber reinforced plastic (“CFRP”). CFRP generally comprisesone or more composite layers or plies laminated together to form asheet, laminate or layup. Each of the composite layers or pliescomprises a reinforcement material and a matrix material. The matrixmaterial surrounds, binds and supports the reinforcement material, andis generally a non-conductive polymer such as an epoxy resin. Foraerospace applications, an aerospace grade resin is used, typicallyhaving four (4) epoxide groups in each epoxy monomer molecule to formmultiple connections. The reinforcement material provides structuralstrength to the matrix material and the CFRP, and generally consists ofstrands of carbon fiber, which are electrically conductive. As usedherein, the term “composite structure” means a structure that ismanufactured, fabricated or assembled, in whole or in part, from one ormore component parts made from composite materials (i.e., compositecomponents) including, without limitation, aerospace structures.

It is desirable to increase the amount of carbon in CFRP to furtherimprove mechanical and/or electrical properties of composite structureswithout increasing weight or disturbing other desirable properties. But,simply increasing the amount of carbon fiber reinforcement material inCFRP does not meet this goal and is not cost efficient. Other forms ofcarbon, such as graphene, which has exceptional mechanical strength andthermal conductivity, would have beneficial effects in compositestructures. Graphene is a hexagonal array of carbon atoms extending overtwo dimensions (i.e., it is one atom thick) that is typically producedin small flakes (or nanoplatelets). Each carbon atom in graphene iscovalently bonded to three other carbon atoms, providing exceptionalstrength. However, mixing graphene into an epoxy resin comprising carbonfibers makes the epoxy resin weaker to strain in every direction becausegraphene will not bond with the carbon fibers and does not interact muchwith the epoxy resin.

Accordingly, there is room for improving the mechanical and electricalproperties of composite structures and related methods for manufacturingcomposite structures that provide advantages over known compositestructures and manufacturing methods.

SUMMARY

The foregoing purposes, as well as others, are achieved by integratingfunctionalized graphene, with amine groups formed on the surface of thegraphene, epoxide groups formed on at least one edge of the grapheneand/or holes formed through the graphene, into CFRP composite materialsas a supplement to or as a replacement for resin matrix material. Theamine groups in the functionalized graphene form strong bonds withepoxide groups in the resin matrix material and the epoxide groups inthe functionalized graphene form strong bonds with amine groups in theresin matrix material, therefore overcoming prior difficulties ofcompositing graphene with aerospace-grade and other resins. Holes may beprovided through the graphene to provide additional edges where epoxidegroups can form and bond to amine groups in the resin matrix material.

In accordance with a product of the disclosure, a composite materialcomprises carbon fiber reinforcement material and a matrix materialcomprising 0.1% to 100% by weight functionalized graphene nanoplateletshaving amine groups formed on a surface of the graphene nanoplateletsand epoxide groups formed on at least one edge of the graphenenanoplatelets. The amine groups have a surface density of about 4.0E10to about 2.0E12 groups per square millimeter or about 0.1% to about 5.0%of the carbon atoms in the graphene nanoplatelets have amine groupsbonded thereto. The epoxide groups have a linear density at the edge ofthe graphene nanoplatelets of about 7,000 to about 700,000 groups permillimeter or about 0.1% to about 10% of carbon atoms at the edge of thegraphene nanoplatelets have epoxide groups bonded thereto. Optionally,holes having a size of about 12-80 carbon atoms or a diameter of about1-2 nanometers may be formed through the graphene nanoplatelets.

In one embodiment of the composite material, the functionalized graphenenanoplatelets are mixed into an aerospace-grade epoxy resin matrixmaterial such that the matrix material has 0.1% to 5.0% by weightfunctionalized graphene nanoplatelets, and the graphene nanoplateletsare present throughout the epoxy resin matrix material. The epoxy resinis a macromolecular complex.

In another embodiment of the composite material, the functionalizedgraphene nanoplatelets are formed as an interlayer between a firstcomposite layer comprising carbon fiber reinforcement materialimpregnated in an aerospace-grade epoxy resin matrix material and asecond composite layer of aerospace-grade epoxy resin matrix materialwith no carbon fiber reinforcement material. The interlayer is amacromolecular complex of the epoxy resin.

In a further embodiment of the composite material, the matrix materialcomprises 100% by weight functionalized graphene nanoplatelets. There isno resin in the matrix material. The composite material comprises onlycarbon fiber reinforcement material and functionalized graphenenanoplatelets. The matrix material in this embodiment is amacromolecular complex of graphene nanoplatelets.

In accordance with a method of the disclosure, a method of increasingstrength, stiffness and modulus of a composite material comprisingcarbon reinforcement fibers and a resin matrix material is provided. Ina first embodiment, functionalized graphene nanoplatelets having aminegroups formed on a surface of the graphene nanoplatelets and epoxidegroups formed on at least one edge of the graphene nanoplatelets aremixed into a resin matrix material to form a graphene-resin mixture. Thegraphene-resin mixture is combined with a plurality of carbonreinforcement fibers to form a prepreg material, and the prepregmaterial is cured to form the composite material with increasedstrength, stiffness and modulus.

In another embodiment of the method, a resin matrix material is combinedwith a plurality of the carbon reinforcement fibers to form a prepregmaterial. Functionalized graphene nanoplatelets having amine groupsformed on a surface of the graphene nanoplatelets and epoxide groupsformed on at least one edge of the graphene nanoplatelets are depositedonto a top surface of the prepreg material to form a grapheneinterlayer. A second layer of the resin matrix material may be laid upon top of the graphene interlayer, and the prepreg material, thegraphene interlayer and the second layer of resin matrix material may becured to form the composite material.

In a further embodiment of the method, a bed of carbon reinforcementfibers is formed, and functionalized graphene nanoplatelets having aminegroups formed on a surface of the graphene nanoplatelets and epoxidegroups formed on at least one edge of the graphene nanoplatelets aredeposited through a top surface of the bed of the carbon reinforcementfibers and penetrate the entire bed of fibers to form a carbonfiber/graphene prepreg material. The carbon fiber/graphene prepregmaterial is then cured to form the composite material.

Composite structures, including aerospace structures, comprisingcomponent parts made with composite materials having the disclosedfunctionalized graphene nanoplatelets, aircraft comprising suchcomposite structures, and methods for making such composite structuresare also considered to be within the scope of the present disclosure.Other objects, features, and advantages of the various embodiments inthe present disclosure will be explained in the following detaileddescription with reference to the appended drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration of a perspective view of an aircraft that mayincorporate the composite materials disclosed herein.

FIG. 2 is an illustration of a typical composite material comprisingcarbon reinforcement fibers and a matrix material.

FIG. 3 is an illustration of one embodiment of an improved compositematerial in accordance with this disclosure.

FIG. 4 is an illustration of another embodiment of an improved compositematerial in accordance with this disclosure.

FIG. 5 is an illustration of yet another embodiment of an improvedcomposite material in accordance with this disclosure.

FIG. 6 is an illustration of one embodiment of a functionalized graphenenanoplatelet that may be used in the improved composite materials ofthis disclosure.

FIG. 7 is an illustration of another embodiment of a functionalizedgraphene nanoplatelet that may be used in the improved compositematerials of this disclosure.

FIG. 8 is an illustration of a flow diagram of an exemplary method forincreasing the strength, modulus and stiffness of a composite material.

FIG. 9 is an illustration of an exemplary manufacturing process linethat may be used in the method shown in FIG. 8.

FIG. 10 is an illustration of a flow diagram of another exemplary methodfor increasing the strength, modulus and stiffness of a compositematerial.

FIG. 11 is an illustration of an exemplary manufacturing process linethat may be used in the method shown in FIG. 10.

FIG. 12 is an illustration of a flow diagram of yet another exemplarymethod for increasing the strength, modulus and stiffness of a compositematerial.

FIG. 13 is an illustration of an exemplary manufacturing process linethat may be used in the method shown in FIG. 12

DETAILED DESCRIPTION

In the following detailed description, various embodiments of compositematerials are described with reference to aerospace structures toillustrate the general principles in the present disclosure. It will berecognized by one skilled in the art that the present disclosure may bepracticed in other analogous applications or environments and/or withother analogous or equivalent variations of the illustrativeembodiments. For example, the composite materials may be used in anyindustry that seeks the benefits of strong and light-weight materials.One of ordinary skill in the art will recognize and appreciate that thecomposite materials and related methods of increasing strength,stiffness and modulus in composite materials of the disclosure can beused in any number of applications involving such vehicles andstructures. It should also be noted that those methods, procedures,components, or functions which are commonly known to persons of ordinaryskill in the field of the disclosure are not described in detail herein.

Referring more particularly to the drawings, FIG. 1 is an illustrationof a perspective view of an exemplary aircraft 10 that may incorporate acomposite structure 26 comprising a composite material augmented withfunctionalized graphene nanoplatelets in accordance with thisdisclosure. As shown in FIG. 1, the aircraft 10 comprises a fuselage 12,a nose 14, a cockpit 16, wings 18, one or more propulsion units 20, avertical tail portion 22, and horizontal tail portions 24. Although theaircraft 10 shown in FIG. 1 is generally representative of a commercialpassenger aircraft having one or more composite structures 26, theteachings of the disclosed embodiments may be applied to other passengeraircraft, cargo aircraft, military aircraft, rotorcraft, and other typesof aircraft or aerial vehicles, as well as aerospace vehicles,satellites, space launch vehicles, rockets, and other aerospacevehicles, as well as boats and other watercraft, trains, automobiles,trucks, buses, or other suitable vehicles or items having compositestructures.

The composite structures 26 may be any type of structure that ismanufactured, fabricated or assembled, in whole or in part, from one ormore component parts made from composite materials. An exemplaryillustration of a typical CFRP-type of composite material 28 is shown inFIG. 2 comprising a single layer of carbon reinforcement fibers 30impregnated with a matrix material 32. The matrix material 32 surrounds,binds and supports the carbon reinforcement fibers 30 and is generally anon-conductive polymer such as an epoxy resin 33. For aerospaceapplications, an aerospace-grade resin is used as the matrix material32, having four (4) epoxide groups in each epoxy monomer molecule toform multiple connections. Typically, the molecules are tri- ortetra-functional epoxies and bifunctional amines. Since one component isat least tri-functional, the result of the extensive epoxy-aminechemical reaction is a dendrimeric structure, which provides strengthand toughness greater than non-aerospace grade bi-functional epoxyresins. Aerospace-grade resins suitable for the composite materials ofthis disclosure should include epoxy-amine resin systems having a curetemperature in the range of about 250° F. to 355° F. Examples of suchresins include, but are not limited to, a toughened epoxy resin 3900-2available from Toray Resin Company, a division of Toray Industries,Inc., Troy, Mich., and the CYCOM® 977-3 and CYCOM® 5320-1 curing resinsavailable from Cytec Industries Inc., West Paterson, N.J.

In view of its exceptional properties, graphene has attracted tremendousresearch interest in recent years, particularly in the field ofelectronics. Graphene can now be made flawlessly or with controlledflaws in a molecular monolayer of unlimited length and width (i.e., itmay be scaled for roll-to-roll processing), with grain size on the orderof 100 nanometers. The controlled flaws can include amino-terminatedgroups (or amine groups) or other chemical functionalization withcontrolled density. Graphene may also be formed with holes having apredetermined size and location. In addition, graphene can now beoxidized by weak acids in whole or in part to form graphene derivatives,such as graphene oxide (GO) and reduced graphene oxide (rGO), havingepoxide groups throughout the graphene, on part of the graphene, or onlyat its edges.

It has been determined that integrating functionalized graphenenanoplatelets (GNP), with amine groups formed on the surface of the GNP,epoxide groups formed on at least one edge of the GNP and/or holesformed through the GNP, into CFRP-type composite materials 28 (like theone shown in FIG. 2) as a supplement to or as a replacement for theresin 33 in the matrix material 32 provides substantial benefits to thestrength, stiffness and modulus of the composite materials 28, whereasthe compositing of non-functionalized GNP includes no chemicalinteractions between any resin 33 in the matrix material 32 and thenon-functionalized GNP, resulting in slip plane formation and decreasedstrength, stiffness and modulus in aerospace-grade resin matrixmaterials 32. FIGS. 3-5 illustrate three exemplary embodiments ofimproved composite materials 28A, 28B, 28C, respectively, wherein thematrix material 32 comprises from about 0.1% to 100% by weightfunctionalized GNP 34. At 100 weight %, the matrix material 32 comprisesno resin 33; only functionalized GNP 34, as shown in FIG. 5.

The functionalized GNP 34 has a generally rectangular shape with edges36 having a length on the order of 10-5,000 nanometers (0.01-5 microns).FIG. 6 is an illustration of a functionalized GNP 34 having added aminegroups 38 on the surface 40 of the functionalized GNP 34, and addedepoxide groups 42 on at least one of the edges 36 of the functionalizedGNP 34.

The amine groups 38 on the surface 40 of the functionalized GNP 34 havea surface density of about 4.0E10 to about 2.0E12 groups per squaremillimeter (about 4.0E4 to about 2.0E6 groups per square micrometer orabout 0.4 to about 2.0 groups per square nanometer). The surface densityof amine groups 38 may also be described as about 0.1% to about 5.0% ofcarbon atoms 37 in the functionalized GNP 34 have amine groups 38 bondedthereto. That is, 1 to 50 carbon atoms 37 per 1,000 carbon atoms have anamine group 38 bonded thereto.

GNP may be functionalized with amine groups 38 on the surface 40 byseveral methods including, for example, the methods disclosed in U.S.Publication No. 2014/0121350 A1 to YOU et al., published May 1, 2014,for preparing a polyimide-graphene composite material, and the methoddisclosed in Matei, Dan G. et al., “Functional Single-Layer GrapheneSheets from Aromatic Monolayers,” Advanced Materials, 2013, 25,4146-4151, WILEY-VCH Verlag GmbH & Co., Weinheim, Germany. In one methodof graphene production, polycyclic aromatic hydrocarbon (PAH) moleculesadsorb to a surface and the interactions of their aromatic rings causethem to form a self-assembled monolayer (SAM). The remainder of eachmolecule beyond its first aromatic ring sticks up from the SAM. Byincluding a second species of PAH with an amine group at some lowconcentration relative to the first PAH species, a SAM with amine groupscan be formed. For example, one out of every 100 aromatic molecules mayhave an extra amine group sticking up out of it. Electron irradiation isused to induce bonds between the aromatic molecules at the surface toform a carbon nanomembrane (CNM). Temperature induced annealing in avacuum or under protective atmosphere will complete the conversion ofCNM into graphene. Other methods of adding amine groups 38 may be used,and any type of amine structure may be added including, for example, 4,4′ diamino diphenyl sulfone;1-(4-aminophenyl)-1,3,3-trimethylindan-5-amine;N,N-bis(4-aminophenyl)benzene-1,4-diamine; 4,4′-methylenedianiline;4,4′-oxydianiline; 3,3′-[1,3-phenylenebis(oxy)]dianiline;4,4′-(9H-fluorene-9,9-diyl)dianiline;4,4′-[1,3-phenylenebis(oxy)]dianiline;4,4′-methylenebis(2-ethylaniline);3,3′-[(2,2-dimethylpropane-1,3-diyl)bis(oxy)]dianiline;3,3′-[1,4-phenylenebis(methylene)]dianiline;4,4′-sulfonylbis(N-methylaniline);4,4′-[1,4-phenylenebis(oxy)]dianiline; 3,3′-sulfonyldianiline; aniline;4,4′-(phenylphosphoryl)dianiline; 3-aminophenol;4,4′-methylenedicyclohexanamine;4,6-diethyl-2-methylbenzene-1,3-diamine;2-(aminomethyl)-2,5,5-trimethylcyclohexanamine; 4,4′-thiodianiline;2,2′-dimethylbiphenyl-4,4′-diamine;N-isopropyl-N′-phenylbenzene-1,4-diamine;N-(1,3-dimethylbutyl)-N′-phenylbenzene-1,4-diamine (Phenyl DMB); andN-cyclohexyl-N′-phenylbenzene-1,4-diamine (Phenyl CH).

The functionalized GNP 34 has a linear density of epoxide groups 42formed on the at least one of the edges 36 of the functionalized GNP 34of about 7,000 to about 700,000 groups per millimeter (or about 0.007 toabout 0.7 groups per nanometer, or about 7 to about 700 per micrometer).The density of epoxide groups 42 may also be represented as about 0.1%to about 10% of carbon atoms at the edges 36 of the functionalized GNP34 have epoxide groups 42 bonded thereto. That is, 1 to 100 carbon atoms37 per 1,000 carbon atoms 37 has an epoxide group 42 bonded thereto.

GNP may be functionalized with additional epoxide groups 42 byoxidation. Graphene can be oxidized by weak acids in whole or in part toform graphene derivatives, such as graphene oxide (GO) and reducedgraphene oxide (rGO), having epoxide groups 42 throughout the graphenederivative, on part of the graphene derivative, or only at its edges 36.The weak acid would first attack the edges 36 of the GNP 34 where thereare hydrogen terminations 43. The amount of oxidation is determined bythe acid strength and exposure time. Examples of weak acids includeformic acid, acetic acid and hydrogen sulfide. It is noted that “weak”does not mean that acid has a high pH. Instead, an acid is described asweak if it is only partially ionized in solution. Exposing theamine-functionalized graphene to a solution of formic acid for up to 30minutes and then cleaning with ethanol may provide the desired densityof epoxide groups 42. Other methods of adding epoxide groups 42 may beused, and any type of epoxide structures may be added including, forexample, 2,2′-[propane-2,2-diylbis(4,1-phenyleneoxymethylene)]dioxirane;2,2′-[methylenebis(4,1-phenyleneoxymethylene)]dioxirane;2,2′-[methylenebis(2,1-phenyleneoxymethylene)]dioxirane;2,2′-[ethane-1,1-diylbis(4,1-phenyleneoxymethylene)]dioxirane; (Bis M);4-(oxiran-2-ylmethoxy)-N,N-bis(oxiran-2-ylmethyl)aniline;2,2′-[thiobis(4,1-phenyleneoxymethylene)]dioxirane;2,2′-[sulfonylbis(4,1-phenyleneoxymethylene)]dioxirane;2,2′-[butane-1,4-diylbis(oxymethylene)]dioxirane;3-(oxiran-2-ylmethoxy)-N,N-bis(oxiran-2-ylmethyl)aniline;2,2′-oxybis(6-oxabicyclo[3.1.0]hexane);2,2′-[1,4-phenylenebis(oxymethylene)]dioxirane;2,2′-[prop-1-ene-1,2-diylbis(4,1-phenyleneoxymethylene)]dioxirane;2,2′-[1,3-phenylenebis(oxymethylene)]dioxirane;2,2′-[cyclohexane-1,2-diylbis(oxymethylene)]dioxirane;2,2′-[(2,2-dichloroethene-1,1-diyl)bis(4,1-phenyleneoxymethylene)]dioxirane;2,2′-[cyclohexane-1,4-diylbis(methyleneoxymethylene)]dioxirane; (Bis I);(Bis AF); (Bis Z); (Bis C); (Bis TMC); (Bis P);2,2′-{propane-2,2-diylbis[(2,6-dibromo-4,1-phenylene)oxymethylene]}dioxirane;7-oxabicyclo[4.1.0]hept-3-ylmethyl7-oxabicyclo[4.1.0]heptane-3-carboxylate.

Optionally, as shown in FIG. 7, functionalized GNP 34 may be furtherfunctionalized by adding at least one hole 44 through the functionalizedGNP 34 to provide additional edges 46 where epoxide groups 42 can form.The at least one hole 44 may be formed by providing energy (such as witha laser) to remove molecules from the SAM prior to formation of the CNM.Preferably, the at least one hole 44 is formed in the functionalized GNP34 to have a substantially circular shape with a diameter of about 1-2nanometers and a size of about 12-80 carbon atoms 37. That is, about12-80 carbon atoms 37 are removed from the functionalized GNP 34 to formthe at least one hole 44 in the functionalized GNP 34. Preferably, eachfunctionalized GNP 34 comprises at least two holes 44. The holes 44 inthe functionalized GNP 34 provide space for molecules in, for example, amatrix material 32 to penetrate the holes 44 and be mechanicallyconstrained by the functionalized GNP 34, thereby further improvingbonding capabilities.

FIG. 8 is an illustration of a flow diagram of an exemplary embodimentof a method 200 of increasing strength, modulus and stiffness of thecomposite material 28 shown in FIG. 2 comprising carbon reinforcementfibers 30 and a matrix material 32 comprising epoxy resin 33. The method200 comprises step 202 of preparing functionalized GNP 34 (see FIG. 6)having amine groups 38 formed on a surface 40 of the functionalized GNP34 and epoxide groups 42 formed on at least one of the edges 36 of thefunctionalized GNP 34 in accordance with the disclosure above. Thefunctionalized GNP 34 should be roughly rectangular in shape with edges36 having a length on the order of about 100-5,000 nanometers (0.1-5microns). Optionally, the functionalized GNP 34 may be provided withholes 44 formed through the functionalized GNP 34 as disclosed above.

In step 204, a matrix material 32 is prepared to be an aerospace-graderesin 33. The matrix material 32 may be prepared having the qualities ofan aerospace-grade resin, or purchased from a supplier ofaerospace-grade resins.

Step 206 of the method 200 comprises mixing the functionalized GNP 34into the matrix material 32 to form a graphene-resin mixture 50 usingequipment and processes known in the art for mixing resins. For example,a mixer having hinge stirrers, paw type stirrers or twisted stirrers, orother types of stirrers may be used. The temperature of the matrixmaterial 32, the speed of the stirrer and the mixing time depend on thetype of equipment used, the type of resin 33, and the density of amine-and epoxide-functionalization of the functionalized GNP 34. Thefunctionalized GNP 34 may be added to the matrix material 32 eitherduring preparation of the matrix material 32 or after the matrixmaterial 32 is prepared to a volume density of about 0.1% to 5.0% of thegraphene-resin mixture 50. The epoxy-amine stoichiometry may be modifiedas appropriate given the addition of the amine groups 38 and epoxidegroups 42 on the functionalized GNP 34. The goal is for thegraphene-resin mixture 50 to have the same ratio of epoxide groups toamine groups as the ratio of epoxide groups to amine groups in thematrix material 32.

Optionally, extreme shear may be provided to the matrix material 32using any commercially-available high shear mixer to cause thefunctionalized GNP 34 to initially flatten and to advance the matrixmaterial 34 to a cure state of approximately 0.1 to lock in the flatfunctionalized GNP 34 state. Cure state is measured from 0 to 1; 1 beinga 100% degree of cure. There are multiple ways to determine the currentcure state of a resin. For example, the amount of energy released when asingle epoxide group reacts with an amine group is known, and the numberof groups per unit mass of a given resin is known. A sample of resin canbe put into a Digital Scanning Calorimeter (DSC) to determine the amountof energy released by the sample over an interval of time (andsimultaneously control the temperature). The determined amount of energyreleased can be divided by the energy per reaction to determine thenumber of reactions that have occurred, and then divided by the numberof groups, or number of possible reactions, in the sample mass todetermine the cure state of the resin. Another way to determine curestate is to place a sample of resin into a DSC, raise the temperatureand wait; that would reveal the amount of energy that was released byadvancing the resin to a 100% (1.0) degree of cure, which can then beused to determine what the resin's cure state had been. Alternatively,the resin's viscosity or modulus can be tested at some specifictemperature. Both of these properties are known to vary with the curestate of the resin. Therefore, the viscosity or modulus as a function ofthe cure state and temperature can be used to determine the cure state.The glass transition temperature also varies with cure state and leavesa signal on certain types of DSC runs, so it can be detectable and usedto infer the cure state.

The method 200 (FIG. 8) further comprises the step 208 of combining thegraphene-resin mixture 50 with a plurality of carbon reinforcementfibers 30 to form a composite prepreg material 52. Preferably, theplurality of carbon reinforcement fibers 30 are pre-impregnated with theuncured graphene-resin mixture 50 using equipment and processes known inthe art for making prepreg materials. The reinforcement fibers 30preferably comprise carbon fibers, carbon-based fibers such as graphitefibers, aramid fibers, fiberglass fibers, glass fibers, KEVLAR® fibers(KEVLAR is a registered trademark of E.I. Du Pont De Nemours and CompanyCorporation of Wilmington, Del.), a combination thereof, or othersuitable carbon or non-carbon fibers. The composite prepreg material 52may comprise the reinforcement fibers 30 in unidirectional (aligned) orfabric (woven) form, impregnated to a desired amount with thegraphene-resin mixture 50. The graphene-resin mixture 50 preferablytransfers stresses between the reinforcement fibers 30 and thus protectsthe reinforcement fibers 30 from mechanical and/or environmentalstresses.

An exemplary illustration of equipment that may be used in step 208 toform the composite prepreg material 52 is shown in FIG. 9. A backingpaper 56 is fed from a backing paper roll 58. The graphene-resin mixture50 is deposited onto the backing paper 56 by any known type of resinapplication device 48 such as a fluid dispenser. The backing paper 56and graphene-resin mixture 50 is fed through a nip 60 between a spreaderbar 61 and a backing bar 62 to form a graphene-resin film 57. The heightof the nip 60 between the spreader bar 61 and the backing bar 62determines the thickness of the graphene-resin film 57. Thegraphene-resin mixture 50 forms a meniscus 64 behind the spreader bar 61because there is too much graphene-resin mixture 50 to be squeezedthrough the nip 60. One or more creels 54 of carbon fiber 30 are fedthrough a spreading comb 66 and through a second set of a spreader 68and a backing bar 69 to produce a bed 70 of aligned carbon fibers on topof the graphene-resin film 57. The bed 70 of aligned carbon fibers 30and the backing paper 56 with graphene-resin film 57 are fed toward apair of nip rollers 72. The nip rollers 72 are heated to decrease theresin viscosity and apply a pressure such that the graphene-resin film57 penetrates the bed 70 of aligned carbon fibers 30 to produce acomposite prepreg material 52.

In step 210 of the method 200, the composite prepreg material 52 iscured with heat and/or pressure sufficient to form the compositematerial 28A (shown in FIG. 3) using equipment and processes known inthe art. Alternatively, the composite prepreg material 52 may be formedinto various shapes to form component parts for composite structures,and then cured.

In the resulting composite material 28A, the amine groups 38 in thefunctionalized GNP 34 form strong bonds with epoxide groups 42 in theresin 33 of the matrix material 32, and epoxide groups 42 in thefunctionalized GNP 34 form strong bonds with amine groups in the resin33 of the matrix material 32. This method results in a nanocompositewherein the resin 33 of the matrix material 32 is a macromoleculecomprised of a base amine monomer (such as 44′DDS), base epoxy monomer(such as Bisphenyl F), and the functionalized GNP 34. The dendrimericstructure of the macromolecule has been replaced with a more complicatedstructure that may contain loops interpenetrating other similarmacromolecules, and when there are holes 44 in the functionalized GNP34, the complicated structure may contain linkages that penetrate theholes 44, either within the same macromolecule or in other similarmacromolecules.

FIG. 10 is an illustration of a flow diagram of another exemplaryembodiment of a method 300 of increasing strength, modulus and stiffnessof the composite material 28 shown in FIG. 2 comprising carbonreinforcement fibers 30 and a matrix material 32 comprising epoxy resin33. The method 300 comprises step 302 of preparing functionalized GNP 34(see FIGS. 6 and 7) having amine groups 38 formed on a surface 40 of thefunctionalized GNP 34 and epoxide groups 42 formed on at least one ofthe edges 36 of the functionalized GNP 34 in accordance with thedisclosure above. The functionalized GNP 34 should be roughlyrectangular in shape with edges 36 having a length on the order of about100-5,000 nanometers (0.1-5 microns). Optionally, the functionalized GNP34 may be provided with holes 44 formed through the functionalized GNP34 as disclosed above.

In step 304, a matrix material 32 is prepared to be an aerospace-graderesin 33. The matrix material 32 may be prepared having the qualities ofan aerospace-grade resin, or purchased from a supplier ofaerospace-grade resins. The matrix material 32 in this embodiment doesnot include any functionalized GNP 34.

The method 300 further comprises the step 306 of combining the matrixmaterial 32 with a plurality of carbon reinforcement fibers 30 usingequipment and processes known in the art to form a composite prepregmaterial 52A. For example, the equipment illustrated in FIG. 9 may beused by applying a matrix material 32 (without any functionalized GNP34) from the application device 48 to form the composite prepregmaterial 52A. In FIG. 11, the prepreg material 52A is shown exiting thenip rollers 72 shown in FIG. 9.

In step 308, referring to the exemplary processing equipment in FIG. 11,functionalized GNP 34 is deposited onto a top surface 74 of the prepregmaterial 52A to form a graphene interlayer 35 (see FIG. 4) by any knownaerial application method for depositing solid flakes, powders, orliquids, such as dusting, dispersion by sonication in a lowvapor-pressure solvent, or pouring or spraying at such a mass rate thatafter the solvent evaporates, the remaining functionalized GNP 34 coversthe desired area on the top surface 74 of the prepreg material 52A. Thegraphene interlayer 35 preferably covers up to about 30% of the area ofthe top surface 74 of the prepreg material 52.

In step 310, an optional second layer 76 of matrix material 32 may belaid on top of the graphene interlayer 35 to form a film 78 to bind thefunctionalized GNP 34. The second layer 76 of matrix material 32 may ormay not include any functionalized GNP 34. The second layer 76 of matrixmaterial 32 is preferably fabricated as a film on a second backing paperthat goes through its own spreader bar that determines its thickness.Then, the second layer 76 is laid onto the graphene interlayer 35 suchthat the film is positioned adjacent the graphene interlayer 35 and thesecond backing paper is exposed. The prepreg material 52A, the grapheneinterlayer 35 and the second layer 76 of matrix material 32 are fedthrough a second nip 80 between a second spreader bar 81 and a secondbacking bar 82, and then toward a pair of heated nip rollers 84 toconsolidate the final prepreg 86. The second backing paper in the secondlayer 76 should then be removed. In step 312, the final prepreg 86 iscured to form the composite material 28B (see FIG. 4) using equipmentand processes known in the art. If the second layer 76 of matrixmaterial 32 includes functionalized GNP 34, then shear could be appliedin the mixer that mixes the functionalized GNP 34 with the matrixmaterial 32.

Applying this method results in a carbon fiber reinforced polymerlaminate with nanocomposite interlayer toughener. The tougheninginterlayer 35 is a macromolecule comprised of the base amine monomer(such as 44′DDS), base epoxy monomer (such as Bisphenyl F), and thefunctionalized GNP 34. The dendrimeric structure of the macromolecule inthe interlayer 35 has been replaced with a more complicated structurethat may contain loops interpenetrating other similar macromolecules. Ifthe functionalized GNP 34 has holes 44, then the complicated structuremay also contain linkages that penetrate the holes 44, either within thesame macromolecule or in other similar macromolecules

FIG. 12 is an illustration of a flow diagram of yet another embodimentof a method 400 of increasing strength, modulus and stiffness of thecomposite material 28 shown in FIG. 2 comprising carbon reinforcementfibers 30 and a matrix material 32. In this method, the matrix material32 comprises 100% functionalized GNP 34 and no resin 33. Thefunctionalized GNP 34 has edges 36 having a length on the order of10-100 nanometers, smaller than the previous embodiments. In step 402,and referring to an exemplary process line shown in FIG. 13, a bed 70 ofcarbon reinforcement fibers 30 is formed on a backing paper 56. Similarto the equipment shown in FIG. 9, one or more creels 54 of carbon fiber30 are fed through a spreading comb 66 and through a second set of aspreader 68 and a backing bar 69 to produce a bed 70 of aligned carbonfibers 30 on top of the backing paper 56. In step 404, functionalizedGNP 34 is prepared as disclosed above, but of a smaller size. In step406, functionalized GNP 34 is deposited onto and through a top surface90 of the bed 70 of carbon reinforcement fibers 30 to penetrate theentire bed 70 of fibers. The carbon reinforcement fibers 30 with thefunctionalized GNP 34 are then passed through a pair of nip rollers 91to compress and form a carbon/graphene prepreg material 92. Thefunctionalized GNP 34 is deposited to a controlled even density, such as60 grams per square meter to 120 grams per square meter. In step 408,the carbon/graphene prepreg material is cured with equipment andprocesses known in the art to form the composite material 28C shown inFIG. 5. The functionalized GNP 34 in this method 400 creates amacromolecule through epoxy-amine reaction.

Many other modifications and variations may of course be devised giventhe above description of various embodiments for implementing theprinciples in the present disclosure. For example, and withoutlimitation, the same technology may be applied to fabric forms ofprepreg materials where the carbon fibers are not aligned. It isintended that all such modifications and variations be considered aswithin the spirit and scope of this disclosure, as defined in thefollowing claims.

The invention claimed is:
 1. A method of increasing strength of a composite material comprising carbon reinforcement fibers and a matrix material, the method comprising: impregnating a plurality of the carbon reinforcement fibers into a first layer of an epoxy resin matrix material having epoxy molecules that are at least tri-functional to form a prepreg material; depositing functionalized graphene nanoplatelets onto a top surface of the prepreg material to forma graphene interlayer that covers up to about 30% of an area of the top surface of the prepreg material, the functionalized graphene nanoplatelets having amine groups formed on a surface of the graphene nanoplatelets and epoxide groups formed on at least one edge of the functionalized graphene nanoplatelets; laying a second layer of the epoxy resin matrix material on top of the graphene interlayer; and curing the prepreg material, the graphene interlayer and the second layer of the epoxy resin matrix material to form the composite material.
 2. The method of claim 1, wherein the graphene nanoplatelets further comprise holes formed through the graphene nanoplatelets.
 3. The method of claim 2, wherein the holes are substantially circular and have a diameter of 1-2 nanometers.
 4. The method of claim 2, wherein the holes have a size of about 12-80 carbon atoms.
 5. The method of claim 1, wherein the amine groups on the surface of the graphene nanoplatelets have a surface density of about 0.4 to about 2.0 groups per square nanometer.
 6. The method of claim 1, wherein about 0.1% to about 5.0% of carbon atoms in the graphene nanoplatelets have amine groups bonded thereto.
 7. The method of claim 1, wherein the graphene nanoplatelets have a linear density of epoxide groups formed on the at least one edge of about 7,000 to about 700,000 groups per millimeter.
 8. The method of claim 1, wherein about 0.1% to about 10% of carbon atoms at the at least one edge of the graphene nanoplatelets have epoxide groups.
 9. The method of claim 1, wherein the prepreg material is cured to a cure state of 0.1.
 10. The method of claim 1, wherein the second layer of the epoxy resin matrix material comprises functionalized graphene nanoplatelets.
 11. The method of claim 1, wherein the second layer of the epoxy resin matrix material is fabricated as a film to bind the functionalized graphene nanoplatelets in the graphene interlayer.
 12. A method of increasing strength of a composite material comprising carbon reinforcement fibers and a matrix material, the method comprising: impregnating a plurality of the carbon reinforcement fibers into a first layer of an epoxy resin matrix material to form a prepreg material; covering up to about 30% of a top surface of the prepreg material with functionalized graphene nanoplatelets to form a graphene interlayer on the top surface of the prepreg material, the functionalized graphene nanoplatelets having amine groups formed on a surface of the graphene nanoplatelets and epoxide groups formed on at least one edge of the graphene nanoplatelets; laying a second layer of the epoxy resin matrix material on top of the graphene interlayer; and curing the prepreg material, the graphene interlayer, and the second layer of the epoxy resin matrix material to form the composite material.
 13. The method of claim 12, wherein the second layer of the epoxy resin matrix material comprises functionalized graphene nanoplatelets.
 14. The method of claim 12, wherein the graphene nanoplatelets further comprise substantially circular holes having a diameter of 1-2 nanometers formed through the graphene nanoplatelets.
 15. The method of claim 12, wherein the amine groups on the surface of the graphene nanoplatelets have a surface density of about 0.4 to about 2.0 groups per square nanometer and the graphene nanoplatelets have a linear density of epoxide groups formed on the at least one edge of about 7,000 to about 700,000 groups per millimeter.
 16. The method of claim 12, wherein about 0.1% to about 5.0% of carbon atoms in the graphene nanoplatelets have amine groups bonded thereto.
 17. The method of claim 12, wherein about 0.1% to about 10% of carbon atoms at the at least one edge of the graphene nanoplatelets have epoxide groups.
 18. The method of claim 15, wherein the prepreg material is cured to a cure state of 0.1.
 19. The method of claim 10, further comprising applying shear to the second layer of the epoxy resin matrix material.
 20. The method of claim 13, further comprising applying shear to the second layer of the epoxy resin matrix material. 